The cryogenic separation of mixtures such as air to produce nitrogen is a well established industrial process. Liquid and vapor are passed in countercurrent contact through a column of a cryogenic rectification plant and the difference in vapor pressure between the oxygen and nitrogen causes nitrogen to concentrate in the vapor and oxygen to concentrate in the liquid. The lower the pressure is in the separation column, the easier is the separation due to vapor pressure differential. Accordingly, the separation for producing product nitrogen is generally carried out at a relatively low pressure.
Often product nitrogen gas is desired at a high pressure. In such situations, the product nitrogen gas is compressed to the desired pressure in a compressor. This compression is costly in terms of energy costs as well as capital costs for the product compressors. Moreover, the compression of the nitrogen product gas may generate impurities such as particulates and these impurities may be detrimental if the nitrogen gas is to be used in an application requiring high purity, such as in the manufacture of semiconductors. In such instances, a further purification step for the nitrogen gas product may be necessary.
Another way of producing high pressure nitrogen product gas is to operate the column of the cryogenic air separation plant at an elevated pressure. In many cases, this is disadvantageous because the column operating pressure required to produce the desired product pressure is higher than that pressure which gives the optimal cycle efficiency, resulting in increased operating costs.
Accordingly, it is an object of this invention to provide a cryogenic rectification system wherein product nitrogen gas may be efficiently produced while avoiding high operating pressures within the cryogenic rectification plant and avoiding the need to compress nitrogen gas product.